Hi everyone So I have an extra ATX PC Power supply unit. I work on embedded systems .... so I kinda need 12V, 3.3V,5V most of the times .. I did some research and found out an ATX PSU has all these voltages .. I am just not sure about the current ratings as the PSU says 3.3V 2A.

I have a device which works on 3.3V and says maximum current rating must be < 250mA .. Can I use this PSU to power up my device .. ?

If it's speced for 2A and you need more than .25A, then yes, you'll be fine.
Amps are pulled, not pushed.
worst case for anything with an atx psu (assuming you don't plug it into the wrong voltage) is that it draws too much current and shuts off the power supply